The present invention relates to a filter device of a three-conductor structure type adapted for use miniature electronic circuits.
There is known a filter device of a three-conductor structure type in which a stripline pattern is disposed between contact surfaces of two dielectric substrates stacked and comprises a plurality of juxtaposed resonator conductors. Such a filter device is of small size and simple, and thus is preferably used in the miniature electronic circuits.
Generally, the conventional filter device of a three-conductor structure type has a frequency response which depends on the size of the resonator conductors, the structure of an external conductor to which the resonator conductors are connected, the configuration and dielectric constant of the substrates or other factors. Therefore, if there are any dispersions in the configuration and dielectric constant of the substrates or the like, the frequency response of the filter may substantially be varied so that it may be deviated from a predetermined frequency range.
In order to avoid the disadvantage, that is, to maintain the frequency response characteristic of the filter within the predetermined frequency range, it has been proposed to use adjusting screws or adjusting devices such as external capacitors. A clamping devices have also been provided for compressing the dielectric material so as to change the electrical parameters of the resonator conductors.
However, the provision of the adjusting means results in the increased size and cost of the filter and therefore these various adjusting means have not always been effective or satisfactory.
On the other hand, a solution to the problem mentioned above is disclosed in U.S. Pat. No. 4,157,517 issued June 5, 1979 to Thomas F. Kneisel et al. In the filter disclosed in this patent, an external conductor provided on the upper surface of an upper substrate is partially removed at a portion positioned above a resonator conductor to reduce the capacitance between the external conductor and the resonator conductor thereby making it possible to adjust the frequency.
With this arrangement, however, since a casing for containing the filter comes into close contact with the upper and lower external surfaces of the filter to reduce the height of the casing, the dielectric substrate comes into direct contact with the casing via the removed portion of the external conductor or an opening. Therefore, after adjusting and assembling of the filter, the distributed capacity may be varied so that the frequency may be deviated.